The present application is the U.S. national stage application of International Application PCT/AU99/00683, filed Aug. 24, 1999, which international application was published on Mar. 2, 2000 as International Publication WO 00/13069 in the English language. The International Application claims the priority of Australian Patent Application PP 5530, filed Aug. 27, 1998.
This invention relates to machines tools and particularly relates to a method of and apparatus for adjusting a collet in a machine tool, such as a computer numerically controlled (CNC) tool cutting and grinding machine.
In CNC tool cutting and grinding machines there are several sources of error which can result in an inaccurately ground workpiece. This invention is principally concerned with compensating for possible runout error in a workpiece held by and extending from a split, draw-back collet.
Conventionally, a split, draw-back collet in a CNC tool cutting and grinding machine has a cylindrical part received in a bore in a collet adaptor or arbor mounted to a rotatable spindle, the collet having a split part at its front end with gripping jaws and a tapered external surface engageable with a complementary tapered surface on the collet adaptor. The collet has a screw-threaded rear end engageable with a complementary screw-threaded front portion of a drawbar extending rearwardly from within the collet adaptor such that when the drawbar is moved rearwardly relative to the collet adaptor, the engagement of the tapered surfaces causes the gripping jaws to tighten on the workpiece to clamp the workpiece securely. This type of collet adaptor, however, can suffer from runout errors of concentricity and angularity. Errors in concentricity occur when the central longitudinal axis of the collet and workpiece is offset from the central axis of the rotatable spindle, and errors in angularity occur when the central longitudinal axis of the collet and clamped workpiece extends at an angle, albeit slight, to the central axis of the rotatable spindle.
It is therefore desirable to provide a method of and apparatus for compensating for runout errors in a split draw-back collet.
It is also desirable to provide an improved collet adaptor which includes an effective means for compensating for errors in runout, particularly errors in angularity.
According to one aspect of the invention, there is provided a collet adaptor for a split, draw-back collet having a cylindrical part for mounting in the collet adaptor and a tapered split part at its front end for gripping a workpiece, the collet adaptor comprising:
a body with a cylindrical bore for receiving the cylindrical part of the collet and a tapered surface at the front end of the adaptor for engagement with the tapered, split part of the collet; and
anti-rotation means for preventing rotation of the collet relative to the collet adaptor, wherein the collet adaptor includes a plurality of adjustment pins engageable with the cylindrical part of the collet at circumferentially spaced locations around the collet to provide a fine adjustment of the angle of the central longitudinal axis of the collet relative to the central longitudinal axis of the collet adaptor.
Preferably, the adjustment pins are spring-loaded and are movable in a substantially radial direction relative to the central axis of the collet adaptor. In a particularly preferred embodiment, each adjustment pin has an enlarged head with a curved surface engaged by a cam surface of an elongate adjustment member which is movable relative to the collet adaptor in a longitudinal direction substantially parallel to the central longitudinal axis of the collet adaptor. Preferably, each elongate adjustment member has a curved end portion engaged by a tapered surface on an adjustable element in the form of an adjustment screw which is radially movable relative to the central longitudinal axis. Thus, a radial movement of the adjustment screw is translated to a longitudinal movement of the elongate adjustment member which, in turn is translated to a fine radial movement of the adjustment pin. Such an arrangement enables a very fine micro adjustment of the angularity of the central longitudinal axis of the collet relative to the central longitudinal axis of the collet adaptor.
The body of the collet adaptor preferably has a recess in its rear end adapted to receive the front end portion of a drawbar to which the rear part of the collet is attachable. In order to permit very fine micro adjustments of the adjustment pins, the drawbar preferably has a flexible elongate intermediate portion between its front end portion and an enlarged rear end portion.
Conventional collet adaptors usually have a frusto-conical rear end portion which is mounted to the headstock of a rotatable spindle by being received in an internally tapered recess in the headstock. This can lead to errors in both concentricity and angularity which are added to similar magnitudes of error in the collet adaptor.
In order to reduce errors caused by the mounting of the collet adaptor to the headstock, the collet adaptor of the present invention preferably has an accurately ground planar rear face which is mounted to an accurately ground planar front face of the spindle headstock. With such a collet adaptor it is possible to substantially eliminate errors in concentricity by using an indicator to indicate the central axis of a clamped workpiece at the front end of the collet and by radially adjusting the position of the collet adaptor to zero the indicator with respect to the central longitudinal axis of the spindle.
According to another aspect of the invention, there is provided a method of compensating for runout errors in a CNC machine having a spindle rotatable about its central longitudinal axis, a collet adaptor, and a split draw-back collet having a cylindrical part mounted in the collet adaptor and a tapered split part at its font end for gripping a workpiece, said method including:
providing the collet adaptor with an accurately ground planar rear surface, an internal tapered surface at its front end, a cylindrical bore to receive the cylindrical part of a collet, anti-rotation means to prevent rotation of the collet relative to the adaptor, and a plurality of adjustment pins engageable with the cylindrical part of the collet;
mounting the collet adaptor and collet to the headstock of a rotatable spindle with the planar rear surface of the collet adaptor adjacent an accurately ground front surface of the spindle headstock;
clamping a workpiece in the collet;
adjusting the radial position of the collet adaptor and collet relative to the central longitudinal axis of the spindle to compensate for errors in concentricity between the central longitudinal axis of the spindle and the central longitudinal axis of the internal tapered surface of the collet adaptor; and
adjusting one or more of said adjustment pins to compensate for errors in angularity between the central longitudinal axis of the workpiece clamped by the collet and the central longitudinal axis of the spindle.
A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
FIG. 1 is a longitudinal section through a collet adaptor assembly in accordance with the invention; and
FIG. 2 is a section on the line Axe2x80x94A of FIG. 1.